Tubular 1D nanostructures

brief introduction

One-dimensional (1D) structured materials are in nanoscale in two dimensions. Examples include nanotubes, nanofibers, nanowires, etc. Electrons are confined within two dimensions, indicating electrons cannot move freely. These 1D nanomaterials can be amorphous or crystalline; single or poly crystalline, and metallic, ceramic, or polymeric similar to the 0D materials. 1D nanomaterials can be chemically pure or impure (such as in doped semiconductor).

Carbon nanotubes

From the beginning of the catalysis, coke formation on catalysts was an important part of the process, and hence, it is rather well documented. With the industrial recovery, this field became much more important since in most cases coke formation is undesirable. The prevention of carbon deposit accumulation is a high priority objective in many processes involving transformation of hydrocarbons. In the latter industrial reactions coke formation can cause not only the deactivation of the catalyst but since it represents a large amount of solid material, it can bring on the blockage of the reactors and deteriorating heat transfer properties. However, because of the numerous different and interesting carbon structures (fibers, cones, tubular structures, etc.) generated as byproduct during the catalytic reactions, the new family of these materials started to be investigated.

The story of the carbon family is grandiose: subtlety in simplicity. The mystery is coming from the different hybridization that carbon atoms can assume. Carbon has four valence electrons. When they are shared equally (sp3), diamond is formed. When three electrons are shared in a plane and one is delocalized between the planes (sp2), the carbon forms graphite.

Since Iijima's discovery, the field of carbon nanotubes (Single- and Multiwall Carbon Nanotubes - SWNTs and MWNTs) has become a separate subject in material science. These materials attracted more and more attention from physicists and chemists. Their surprisingly new and unique physical and chemical properties generated tremendous interest. Let us note that carbon nanotubes are also considered as members of the recently discovered carbon allotropes, the fullerenes, for which the Nobel Prize was given in 1996 to Curl, Kroto and Smalley.

Titanate nanotubes

The discovery of carbon nanotubes encouraged researchers to widen their knowledge in the synthesis and characterization of the nanotubular materials. In the last decades several studies have been published about the synthesis and characterization of different inorganic nanotubes. Titanium oxide based nanotubes have became a particular research subject for their applications to photocatalytic, biomedical, energy storing and semiconductor devices, to name a few.
The first sodium-trititanate nanotube synthesis was showed by Kasuga. The procedure was the hydrothermal treatment of TiO2 powders in 10 M NaOH solution and needle-shaped nanotubes were obtained after the reaction.
Titanate nanotubes have been used for heterogeneous catalytic studies as support. The Au decorated titanate nanotubes could be promising catalysts in the low-temperature water- gas shift reaction. Platinum nanoparticles can improve this behavior as well. Pd(II)/titanate nanotubes have high activity and selectivity to the double-bond migration for example in the isomerization of allylbenzene. The photocatalytic activity of trititanate nanotubes is weaker than that of anatase or other titanium-oxide nanocrystals because of the distorted structure and wider band gap. Thus, several studies were published about the photosensitization of the titanate nanotubes. One possible sensitizer is cadmium sulphide, a versatile semiconductor with tunable 2.4–4.2 eV band gap which can be obtained as layered assembly or as core/shell heterostructure. The photocatalytic activity could be improved by framework doping using iron or nitrogen dopants as well.
Metal ion incorporation into titanate nanotubes will definitely change their properties, however, as a result of the metal ion insertion the structure of the nanotubes can be strongly influenced as well.